The present invention relates to a system and method for locating anatomical structures within biological tissue. More particularly, the invention relates to a system and method for locating anatomical structures such as blood vessels in a mammalian body by utilizing equipment sensitive to the unique absorption and scattering characteristics of the target structure, such as blood. Further, the present invention provides a system and method to enhance the contrast between a target structure, such as a blood vessel, and its surrounding tissue.
Every day in the United States, many hundreds-of-thousands of medical procedures involving the puncturing of blood vessels are performed. Venipuncture, as it is known, is required in order to administer emergency fluids, blood components, and anesthetics during operations, or to allow the drawing of blood for biochemical analysis. Venipuncture, which is often the rate-limiting step when administering intravenous compounds, can take as long as a half hour with a typical patient or longer when the patient is a neonate, infant, geriatric, obese or burn patient. Notwithstanding the enormous financial burden on our society as a whole because operating rooms and health-care providers must wait as an intravenous line is placed, the delay in placing an intravenous line can in fact be life threatening. Furthermore, there is a high morbidity associated with multiple venipunctures caused by the clinician's failure to locate the vessel.
The reason venipuncture is sometimes difficult to do is that the blood vessels are often located relatively deep within the tissue which, because of its absorptive and scattering optical properties, makes visualization of the blood vessel impossible under normal conditions. Furthermore, the situation is made worse by the fact that the vessel may spasm and constrict if it is manipulated too much. Consequently, health care providers have a need to visualize blood vessels in real-time during venipuncture in order to reduce the risk to the patient, save time and reduce the cost of the procedure. Furthermore, reducing the time of the procedure limits the providers' exposure to a potentially contaminated needle. Finally, visualization of vascular tissue can provide important diagnostic and therapeutic information about certain diseases such as thromboses, cancers or vascular malformations.
In the mid-1970's an instrument was devised that purportedly provided surgeons with the ability of visualizing superficial blood vessels. It consisted of a visible light source which, when pressed up against the skin, transilluminated the subcutaneous tissue and aided in the visualization of superficial blood vessels. The blood-vessel transilluminator made use of the different absorption properties of blood and tissue. Because blood strongly absorbs certain wavelengths of light, while fat and skin absorb other wavelengths, a health-care provider purportedly could visually distinguish the position of the subcutaneous blood vessel with the naked eye. The transilluminator has essentially fallen into disuse because it fails to provide enough contrast between the blood vessel and tissue to be of use other than for venipuncture of superficial vessels. Furthermore, some versions of the blood-vessel transilluminator caused thermal damage to the patient.
The transilluminator's failure revealed that high contrast was of critical importance to medical personnel. Consequently, several references proposed using an illumination wavelength which penetrates surface tissue to a depth of the deep vessels but which is also highly absorbed by the blood. See, e.g., Cheong, W-F, et al., "A Review of the Optical Properties of Biological Tissues," IEEE Journ. Quant. Elec., 26:2166-2185 (1990). These references, however, did not disclose efficient means of eliminating detection of scattered light from areas outside the vessel region (i.e., off angle light). Nor did they disclose the elimination of detection of polychromatic white noise, such as from ambient room light or from a polychromatic light source. Later devices only employed a subtraction technique using expensive digital processing and cumbersome computer analysis to eliminate unwanted scattered waves. Furthermore, these devices did not disclose a method of noise reduction for use with a white light source, but rather relied on use of a monochromatic laser light source to reduce polychromatic noise. Accordingly, there was a need for a contrast enhancement device usable with a polychromatic light source or in a polychromatic clinical environment.
Most importantly, electromagnetic imaging devices have used transmitted rather than reflected light to construct their image. Such systems house the image detector and the light source on either side of the patient rather than side by side in a single integral unit. Such an arrangement unfortunately does not allow for convenient same-side illuminating and detecting such as in the form of a single unit goggle or scanning device. Accordingly, manipulation of many of these devices along with the patient required multiple clinical personnel. Moreover, these references in fact teach away from the use of any scattered light to create an image, including reflected light. Instead, these devices seek to eliminate all scattered light from detection since such light was thought not to carry any image information.